Apparatus for and method of mining a subterranean ore deposit



April 22, 1969 H. PFEFFERLE 3,439,953

APPARATUS FOR AND METHOD OF MINING A SUBTERRANEAN ORE DEPOSIT Filed May 23. 1967 Sheet of 2 'co/ye P/e ffer/e I N VEN TOR.

April 22, 1969 G. H. PFEFFERLE 3,439,953.

APPARATUS FOR AND METHOD OF MINING A SUBTERRANEAN ORE DEPOSIT Filed May 23. 1967 Sheet 3 of 2 JNVENTOR.

ATTORNEYS I I y q 55 -5 MI United States Patent US. (:1. 299-17 3 Claims ABSTRACT OF THE DISCLOSURE Apparatus for and a method of mining a subterranean ore deposit through a well bore where the ore is eroded from the ore matrix by laterally directed jets of water. The mining operation is started with the well filled with water. After the cavity formed by the eroding jet streams reaches a size such that the jet streams can no longer erode the ore efficiently, the water level in the well is lowered to a point just above the lower end of the eductor tube through which the ore and water slurry is removed. The mining operation then continues with the eroding jet streams traveling through air. The slurry is removed from the well through the eductor tube by an air lift system. The location of the air-water interface is determined by a vertically movable small diameter tubing string.

This invention relates generally to hydraulic mining and, in particular, to apparatus for and a method of mining lgydraulically a subterranean ore deposit through a well ore.

Subterranean ore deposits have been found that cannot be mined by conventional mining methods or by dredging. An example of such an ore body is the phosphate deposit found in eastern North Carolina. The phosphate formation or matrix occurs at depths ranging from 150-300 feet below the Surface, well beyond the reach of the dredges presently available. Conventional mining methods cannot be used because the water table is encountered usually from 5 to feet below the surface.

One method that has been used to mine this phosphate ore is referred to as the air-bubble method. This is a hydraulic mining method where a well bore is sunk from the surface into the phosphate ore body or matrix. The well is cased down to a point just above the phosphate ore. There is a section of overburden above the phosphate ore that is substantially water and air impervious. The casing is sealed to this section of overburden. An eductor tube is located in the casing through which the ore and water slurry produced by the hydraulic mining is removed from the well. A jetting pipe string is also located in the casing with nozzles that direct a stream of water laterally against the ore body. To permit the laterally directed streams of water to strike the walls of the well bore with as much force as possible, an air bubble is established that lowers the water level in the well bore to a point below the jet outlet. The jetting action occurs above this air-water interface and the ore eroded from the matrix and the jet water flow downwardly to the bottom of the well bore as a slurry. It is removed from the well through the eductor tube by the pressure of the air bubble.

One of the problems with this method, particularly at the commencement of the operation, is the danger that the air-water interface will drop below the lower end of the eductor tube. Should this occur, there would be a rapid drop in pressure in the well bore and the pressure differential across the casing seal will more than likely destroy the seal. Without a casing seal, it is usually not practical to continue the mining operation. Also, the overburden, when relieved of supporting pressure, will likely subside to fill the mined cavity with possible loss of equipment and premature well abandonment.

ice

Another problem with this system is the high pressure at which the air-bubble must be maintained to hold the air-water interface at the desired level and to also provide sutficient energy to lift the ore and water slurry to the surface through the eductor tube. This increases the pressure differential the casing seal is required to withstand.

It is an object of this invention to provide apparatus for and a method of hydraulically mining a subterranean ore deposit that greatly reduces the possibility of a sudden drop in the pressure in the well bore because the airwater interface drops below the end of the eductor tube.

It is another object of this invention to provide apparatus for and a method of hydraulically mining a subterranean ore deposit that permits dewatering the well to provide an air-bubble without applying excessive pressure to the casing seal and open hole.

It is another object of this invention to provide apparatus for and a method of hydraulically mining a subterranean ore deposit that reduces the pressure at which the air-bubble must be maintained to hold the airwater interface at the desired level.

It is another object of this invention to provide apparatus for such a hydraulic mining method that will allow the air-water interface to be accurately located quickly and easily.

These and other objects, advantages and features of this invention will be apparent to those skilled in the art from a consideration of this specification, attached drawings, and appended claims.

The preferred apparatus and method for practicing this invention will now be described in connection with the attached drawings in which,

FIGURE 1 is a vertical sectional view through a well bore equipped with the apparatus of this invention;

FIGURE 2 is a vertical sectional view of an alternate embodiment of the apparatus;

FIGURE 3 is an enlarged view of the nozzle assembly employed to direct two streams of water laterally against the ore body or matrix; and

FIGURE 4 is a view taken from the view of FIG- URE 1 showing the well bore at the beginning of the mining operation.

The equipment shown in FIGURE 1 is installed as follows: Conductor casing 10 is driven into the ground to refusal. Alternately, a drill bit can be employed to drill into the earth a short distance after which the conductor pipe is placed in the hole and cemented in place. Often the ore deposit lies beneath marshy or swampy land or beneath a river bottom and the conductor pipe keeps this water out of the well bore as it is being drilled.

Well bore 11 is drilled through conductor casing 10 to a point in overburden 12 just above ore matrix or body 14. Casing 15, the water string, is then lowered into the hole and driven downwardly until it meets substantial resistance. Usually, this is sufficient to create a seal that will keep surface water from migrating downwardly into the cavity which will be formed in the ore body, as the ore is removed. It will also be sufficient to prevent air and water from escaping from the cavity in the ore body during the mining operation.

The well bore is then extended below the water string into the ore matrix. Usually, this section of the well bore, shown in dotted lines in FIGURE 1, and designated 11a is terminated above underlying non-productive formations to avoid exposing them since they may not be impervious to water or they maybe water hearing. In either case, the water-to-air balance that needs to be maintained during the mining operation would be upset. Before in stalling any equipment in the well, preferably, portion 11a 0? the well bore is underreamed to a larger diameter, as indicated by dotted line 19 in FIGURE 1. By enlarging the hole in this manner, jet nozzle assembly 20, shown in FIGURE 3, can be equipped with longer nozzles. Further, it has been found that by enlarging the hole in this manner production is increased from the ore bearing formation by a substantial precentage during the initial portions of the operation of the process.

After the well bore has been extended into the ore matrix, and preferably enlarged, eductor tube 16, jet string 17, and air-water interface indicator 18 are made up and installed in the well. Usually, these pipe strings are lengths of pipe connected together. The three strings extend through openings provided therefor in production head 22, which is attached to the top of water string 15. Stuffing boxes 23, 24, and seal between the production head and the pipe strings. Stuffing boxes 23 and 24 are designed to permit strings 16 and 17 to be reciprocated and rotated. Stuffing box 25 usually is designed to support the indicator tube 18 and permit reciprocation. The equipment for reciprocating and rotating the strings is not shown. Any well known hoisting and rotating equipment can be used for this purpose.

Attached to the bottom of jet string 17 is nozzle assembly 20. As shown in FIGURE 3, the assembly includes body 26, which has two arcuate arms 26a and 2611. These arms are bowed so that their ends are facing horizontally in opposite directions. Mounted on the end of arms 26a and 26b are nozzles 27a and 27]), respectively. With this nozzle assembly, water pumped down the jet string is directed laterally in both directions by the two nozzles and the reactive forces produced by the nozzles are balanced. Preferably, the jet string can be rotated, as well as reciprocated, so the streams of water produced by the nozzles can be played over substantially all the face of the ore body that is exposed by the well bore. The flow of water into the jet strings is controlled at the surface by valve 28.

Eductor tube 16 is positioned with its lower end fairly close to the bottom of the hole. The flow from the eductor tube is controlled by valve 29.

In accordance with this invention, the mining operation is begun with the well bore filled with water. It has been found that the streams of water produced by nozzle assembly 20 will erode the ore from the ore matrix eificiently in the early stages of the mining operation, even though the eroding streams pass through water in reaching the walls. This is one of the important advantages of the mining method of this invention, for it reduces considerably the danger of a sudden drop in pressure occurring in the cavity. As explained above, such a pressure drop causes an increase in the differential pressure across the seal between the water string and the overburden, and the increase may be suflicient to break down the seal or cause overburden subsidence and the cavity to collapse.

By starting with a hole full of water this danger is substantially eliminated. For example, assume that in the beginning the water was displaced from the hole, as shown in FIGURE 4, so that the jet streams would pass through air in reaching the walls of the hole as in the heretofore used air-bubble method. This would require that interface A between the air and water be lowered to a point just above the lower end of eductor tube 16, as shown in FIGURE 1. This would leave a water seal around the lower end of the eductor tube of a height a. Initially, this is a small volume of water.

For example, assume that the underreamed hole is 28 inches in diameter or 2.3 feet. If the eductor tube extends into this water zone 6 inches, the water seal above the end of the production zone amounts to 2.14 cubic feet of water. If water is being pumped through the nozzles at the rate of 900 g.p.m. (120 cu ft./min.), which produces an output slurry of 950 g.p.m. (127 cu. ft./min.), the difference in input and output volume is 7 cubic feet a minute. This is equivalent to a height of 1.64 feet in the 2.3 foot diameter well bore. Consequently, a variation sufficient to drop the water level down to the end of the eductor tube could occur in 30 seconds. Thus, control of the air-water interface is extremely difficult in the initial stages of the operation.

In accordance with this invention, however, the mining operation is initially started with the hole filled with water. This is continued until the solid contents of the produced slurry decreases materially. At this point, a cavity of substantial diameter has been formed in the ore matrix. The well can now be dewatered and an air environment established through which the jets can continue to erode away the walls of the ore matrix with increased efficiency, for the danger of a blow out around the bottom of the eductor tube is substantially reduced. For example, experience has shown that approximately 600 tons of ore can be produced with the hole filled with water.

After producing 600 tons (about 12,000 cu. ft.), the diameter of the hole is approximately 23 feet. If the angle of repose of the material (angle B) is 10, the theoretical shape of the cavity after the 600 tons is produced is somewhat as shown in FIGURE 1. The volume above the production string, with an air-water interface six inches above the lower end of the eductor tube, is 161 cubic feet. With the same input and output variation, 7 cu. ft./min., it will require 23 minutes to change the air-water interface six inches. If the permissible variation in the level of the air-water interface is three inches then it will require 14.3 minutes for the air-water interface to drop three inches, even if solid material were not produced. Thus, there is enough time to correct the flow rate even after the air-water interface approaches within three inches of the bottom of the eductor tube. Thus, in accordance with this invention, when the hole is dewatered for more efficient operation, it is of such a size that the airwater interface level can be easily controlled with substantially no danger of it dropping to the point where there will be a sudden decrease in pressure in the cavity.

Air is injected into water string 15 through the line 31 to displace the water from the well bore and the cavity formed. This line is connected through a manifold 32 to receiver 33. Air is supplied to the receiver through line 34 from a source of high pressure air, such as a compressor. The air enters water string 15 and forces the water downwardly in the casing and out through the eductor tube. To avoid creating a destructively high differential pressure across the casing seal, the dewatering process should be started with a very low air pressure as indicated on gage 35 and gradually increased as the water level falls in water string 15. To determine when the air-water interface reaches the desired level, the lower end of the interface indicator line 18 is positioned the desired height above the lower end of the eductor tube, that is, at the height the air-water interface is to be maintained. When air begins to flow through this line, the interface will have passed the end of the indicator tube. The injection of air can then be stopped and mining operations resumed. Now the jet streams produced by the nozzles will exert their full force against the sides of the cavity.

As explained above, it is important to keep the differential pressure existing across the casing seal as low as possible. When the fluid pressure in the cavity, whether water or air, is used to force the slurry in the bottom of the cavity up the eductor tube, it must be equal to the hydrostatic head of the fluid in the eductor tube plus the frictional head losses of the slurry as it flows upwardly through the tube and the surface fittings. Since this pressure may be substantially greater than hydrostatic pressure, it may tax the ability of the casing seal. To permit the pressure of the air and water in the cavity to be reduced, a separate air lift system is provided for removing the slurry.

The air lift system includes air line 38, which extends into eductor 16 through elbow 39. The lower end of the air lift tube is located a calculated distance above the lower end of the eductor tube. It is provided with perforated foot piece 40, through which the air passes to enter the eductor tube and air lift the slurry upwardly and out of the tube. By aerating the column of slurry in the eductor tube, between the lower end of the air lift tube and the surface, the pressure exerted by the total column of fluid in the eductor tube is reduced. It also provides a much more efiicient means of removing the slurry from the cavity. The air provided for lifting is controlled by valve 41. The air lift system can also be employed during the dewatering step described above to reduce the air pressure build-up through line 31, further reducing the possibility of exerting an excessive pressure differential across the casing seal.

Should the air lift be removing slurry faster than it is being made, make-up water can be added through line 45 and valve 46. Air-water interface indicator line 18 is of small diameter so that even though its lower end is located above the water, and valve 18a is open the amount of air that can flow through it will not cause a quick drop in the pressure in the cavity. Usually, the air-water interface is maintained just above the end of the line. Should the air-water interface drop there will be a surface indication of this and steps can be taken to balance the flow rate, either by cutting down on the rate the slurry is being removed by the air lift or by adding make-up water through line 45. If the air-water interface rises, however, there would be no surface indication. Therefore, in accordance with this invention indicator tube 18 is arranged to be reciprocated through stuffing box 25. Periodically then, the indicator line can be raised until air flows through it, indicating that it has reached the top of the water level in the cavity. Knowing its initial position and the amount it is raised relative to the eductor tube, the air-water interface can be located accurately. If it is too high, more air could be supplied to the air-lift system to increase the rate the slurry is removed or the amount of water being supplied to the nozzles could be reduced or both.

FIGURE 2 shows an alternate embodiment of the apparatus of this invention. In this embodiment, the lower end of eductor tube 16' is provided with venturi tube 50. Extending along the side of eductor tube 16' is second jet string 51. This jet string would normally be clamped to the eductor tube string so that its lower end and nozzle assembly 52 attached to it is accurately positioned relative to the end of the eductor tube. Nozzle assembly 52 includes nozzle 53 which with venturi 50 forms an ejector for pumping water and ore slurry 54 in the bottom of the hole upwardly into the eductor tube. This arrangement is particularly advantageous Where the ore is eroded from the ore matrix in large chunks which need a boost into the eductor tube.

Nozzle assembly 52 is also provided with second nozzle 55. It serves principally as a stirring nozzle to agitate the slurry and keep the ore from settling out as it collects in the bottom of the cavity.

From the foregoing, it will be seen that this invention is one well adapted to attain all of the ends and objects hereinabove set forth, together with other advantages which are obvious and which are inherent to the apparatus and method.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.

As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

The invention having been described, what is claimed is:

1. A method of mining a subterranean ore deposit located beneath a subsantially air and water impervious section of overburden comprising disposing a casing in sealing engagement with the overburden at a point above the ore deposit and extending upwardly therefrom to the surface, forming an open hole below the casing into the ore deposit, maintaining the open hole filled with wa er, conducting a stream of wa er from the surface and directing it laterally through said water in said open hole against the side wall of the open hole below the casing to erode ore from the ore deposit and to creae a slurry of ore and water in the open hole below the casing, educting the ore and water slurry to the surface as it is formed, continuing to thus remove ore until a cavity of greater diameter than said open hole is formed in the open hole below thecasing as a result of the ore removal, the size of said cavity being such that the laterally directed stream of water cannot travel through the water with which the cavity is filled with sufiicient force to efiiciently erode the ore from the ore deposit, and supplying air to the casing at the surface at a pressure sufficient to lower the water level in the cavity until the laterally directed stream of water travels through air against the ore deposit to permit the mining operation to continue efficiently.

2. The method of claim 1 in which the slurry is educted by injecting air into the water and ore slurry to lift the slurry to the surface to reduce the air pressure required to maintain the water level in the cavity at the desired level.

3. The method of claim 2 having the further step of injecting air to educt water from the cavity as air is supplied to the casing to lower the water level in the cavity.

References Cited UNITED STATES PATENTS 931,057 8/1909 Goldsmith 2995 1,960,932 5/1934 Tracy 299-4 2,518,591 8/1950 Aston et a1. 175422 X 2,720,381 10/1955 Quick l67 X 2,745,647 5/1956 Gilmore 2995 2,772,868 12/1956 Brandt 2995 3,030,086 4/1962 Donaldson et al. -422 X ERNEST R. PURSER, Primary Examiner.

US. Cl. X.R. 

